In recent years, the resolution of the scanning electron microscope has been so improved that it has been frequently used at low acceleration voltages, especially for observation of semiconductors so as to avoid damaging the semiconductor with an electron beam.
In the ordinary secondary electron detecting apparatus used in the scanning electron microscope, the secondary electrons emitted from a sample are detected by means of a secondary electron detector through an objective lens magnetic field for focusing the electron beam which irradiates and bombards the sample. A photoelectron multiplication type detector is generally used as the secondary electron detector. This is because that detector has a high detection sensitivity. Since a high voltage for attracting the secondary electrons is applied to the front end of the secondary electron detector, however, it obstructs deflection of the electron beam with the consequent disadvantage that the electron beam is misaligned.
This problem is solved by positioning the secondary electron detector apart from the electron beam. However, this positioning raises another problem that the secondary electrons cannot be efficiently detected.